Lung cancer causes more deaths from cancer worldwide than any other form of cancer (Goodman, G. E., Thorax 57:994-999 (2002)). In the United States, lung cancer is the primary cause of cancer death among both men and women. In 2007, the death rate from lung cancer was an estimated 160,390 deaths, exceeding the combined total for breast, prostate and colon cancer (America Cancer Society, www.cancer.org). Lung cancer is also the leading cause of cancer death in all European countries and is rapidly increasing in developing countries. While environmental factors, such as lifestyle factors (e.g., smoking) and dietary factors, play an important role in lung cancer, genetic factors also contribute to the disease. For example, a family of enzymes responsible for carcinogen activation, degradation and subsequent DNA repair have been implicated in susceptibility to lung cancer. In addition, an increased risk to familial members outside of the nuclear family has been shown by deCODE geneticists by analysing all lung cancer cases diagnosed in Iceland over 48 years. This increased risk could not be entirely accounted for by smoking indicating that genetic variants may predispose certain individuals to lung cancer (Jonsson et. al., JAMA 292(24):2977-83 (2004); Amundadottir et. al., PLoS Med. 1(3):e65 (2004)).
The five-year survival rate among all lung cancer patients, regardless of the stage of disease at diagnosis, is only 13%. This contrasts with a five-year survival rate of 46% among cases detected while the disease is still localized. However, only 16% of lung cancers are discovered before the disease has spread. Early detection is difficult as clinical symptoms are often not observed until the disease has reached an advanced stage. Currently, diagnosis is aided by the use of chest x-rays, analysis of the type of cells contained in sputum and fiberoptic examination of the bronchial passages. Treatment regimens are determined by the type and stage of the cancer, and include surgery, radiation therapy and/or chemotherapy. In spite of considerable research into therapies for this and other cancers, lung cancer remains difficult to diagnose and treat effectively. Accordingly, there is a great need in the art for improved methods for detecting and treating such cancers.
Environmental Risk Factors for Lung Cancer:
Smoking of tobacco products, and in particular cigarettes, is the largest known risk factor lung cancer with a global attributable proportion estimated to be approximately 90% in men and 80% in women. Although the risk of lung cancer associated with tobacco smoking is strongly related to duration of smoking, and declines with increasing time from cessation, the estimated lifetime risk of lung cancer among former smokers remains high, ranging from approximately 6% in smokers who give up at the age of 50, to 10% for smokers who give up at age 60, compared to 15% for lifelong smokers and less than 1% in never-smokers (Peto et al. 2000 BMJ, 321, 323-32, Brennan, et al. 2006 Am J Epidemiol 164, 1233-1241). In populations where the large majority of smokers have quit smoking, such as men in the US and UK, the majority of lung cancer cases now occurs among ex-smokers (Doll et al. 1994 BMJ 309, 901-911, Zhu et al. 2001 Cancer Res, 61, 7825-7829). This emphasizes the importance of developing alternative prevention measures for lung cancer including the identification of high risk subgroups.
Genetic Risk Factors for Lung Cancer:
Notably, only about 15% of lifelong smokers will develop lung cancer by the age of 75, and approximately 5 to 10% of lifetime smokers will develop another tobacco-related cancer (Kjaerheim et al. 1998 Cancer Causes Control 9, 99-108). A possible explanation for this large differences in risk for individuals with similar level of tobacco exposures could be that genetic factors play a determining role in lung cancer susceptibility (Spitz et al. 2005 J Clin Oncol 23, 267-275). Identifying genes, which influence the risk of lung cancer could be important for several aspects of management of the disease.
Segregation analyses predict that the majority of genetic risk for lung cancer is most likely to be polygenic in nature, with multiple risk alleles that confer low to moderate risk and which may interact with each other and with environmental risk factors. Many studies have investigated lung cancer susceptibility based on the presence of low-penetrance, high-frequency single nucleotide polymorphisms in candidate genes belonging to specific metabolic pathways. Genetic polymorphisms of xenobiotic metabolism, DNA repair, cell-cycle control, immunity, addiction and nutritional status have been described as promising candidates but have in many cases proven difficult to confirm (Hung et al. 2005 J Natl Cancer Inst 97, 567-576, Hung et al. 2006 Cancer Res 66, 8280-8286, Landi et al. 2006 Carcinogenesis, in press, Brennan et al. 2005 Lancet 366, 1558-60, Hung et al. 2007 Carcinogenesis 28, 1334-40, Campa et al. 2007 Cancer Causes Control 18, 449-455, Gemignani et al. 2007 Carcinogenesis 28(6), 1287-93, Hall et al. 2007 Carcinogenesis 28, 665-671, Campa et al. 2005 Cancer Epidemiol Biomarkers Prev 14, 2457-2458, Campa et al. 2005 Cancer Epidemiol Biomarkers Prev 14, 538-539, Hashibe et al. 2006 Cancer Epidemiol Biomarkers Prev 15, 696-703).
For cancers that show a familial risk of around two-fold such as lung cancer (Jonsson et al. 2004 JAMA 292, 2977-2983, Li and Hemminki 2005 Lung Cancer 47, 301-307, Goldgar et al. 1994 J Natl Cancer Inst 86, 1600-1608), the majority of cases will arise from approximately 10%-15% of the population at greatest risk (Pharoah et al. 2002 NatGenet 31, 33-36). The identification of common genetic variants that affect the risk of lung cancer may enable the identification of individuals who are at a very high risk because of their increased genetic susceptibility or, in the case of genes related to nicotine metabolism, because of their inability to quit smoking. Such findings could potentially lead to chemoprevention programs for high risk individuals, and are especially of importance given the high residual risk that remains among ex-smokers, among whom the majority of lung cancers in the US and Europe now occur.